41 



used, then the place for the micrometers is between the two 

 lenses. 



To find the value of the lines in the eye-piece micrometer, 

 a glass micrometer is laid on the stage of the microscope, 

 which is divided into 1-lOOth and 1-lOOOth of an inch, when 

 the two micrometers are brought into focus with a low power, 

 say a one inch objective, then if every division of 1-lOOth of 

 an inch in the stage micrometer coincides with ten in that of 

 the eye-piece, it is clear that the lines in the eye-piece micro- 

 meter are 1-lOOOth of an inch, the stage micrometer can now 

 be removed, and every object corresponding to the eye-piece 

 micrometer, under a one inch object glass can be measured to 

 1-1 000th of an inch. 



If one of the 1-lOOOth of an inch of the eye-piece micro- 

 meter be divided into four parts by the stage micrometer then 

 every one of these divisions would be l-400l)th of an inch. If 

 with any other power it is found that 1-lOOOth of an inch on 

 the stage micrometer coincides with ten spaces of the eye- 

 pieces micrometer, then the value of each division in the eye- 

 piece micrometer would be l-10,000th of an inch. In prac- 

 tice it will be found when high numbers are being observed, 

 and the stage movement too coarse, it will be necessary to use 

 the small miiled-head screw at the end of the eye-piece micro- 

 meter, so as to bring the lines in each micrometer accurately 

 over each other. 



For the information of those who take an interest in this 

 subject, I lay on the table the two volumes of Professor 

 Smith's Synopsis of British Diatomacese. In these volumes 

 all the objects are drawn to a scale of 10,000 to the inch, 

 multiplied 400 diameters. Also papers containing some par- 

 ticulars of MoUer's Diatom Typen Plate, and Robert's Band 

 of Test Lines, with Mr. Webb's description of diamond writing 

 and engraving. 



